It's possible someone on the plane, seeing Alpizar acting up, said "he might have a bomb."
I don't think how dangerous it can seem that he left the plane so agitated. The airlines have a safety policy of not putting any luggage on the plane from a passenger who isn't on the flight. Its not just the bags he brought with him but the bags that he could have checked.
I would bet that certain profiles of behavior have been looked at with varying degrees of risk and protocols established. Common sense would dictate that the fact a man has disembarked in a hurry after boarding a plane, has refused to be detained when questioned and reached for a hidden object must have placed this man in the highest risk possible.